Washing of a steel sheet is necessary as a pretreatment before a surface treatment such as coating of the surface of the steel sheet, and is a very large factor to determine the quality of the product. Examples of the stain attached to the surface of a steel sheet include oil stains of rolling oil that attaches during cold rolling and rust preventive oil. In the cold rolling of a steel sheet in recent years, there has been used rolling oil that is suitable for improving the mill cleaning property and the productivity, and an excellent washing property for the rolling oil has been required.
Reduction of carbon dioxide emission has also been required as the environmental awareness increases, and it is not an exception in the iron and steel production. Accordingly, there has been attempted the development of a detergent that exhibits an excellent washing property even at a lower temperature than a conventional washing temperature of 70° C. to 90° C. in the washing of a steel sheet in order to reduce the energy cost that is related to the carbon dioxide emission.
JP-A-2011-117043 discloses an alkali detergent for a steel sheet that contains an alkali agent, a specified nonionic surfactant, a chelating agent, and water to efficiently wash away the stain on a steel sheet and suppress the foaming while suppressing slipping of the steel sheet on a roll at a low washing temperature of 40° C. to 60° C.